In a article by Felix Hong (Sixth
Newsletter of Molecular Electronics and BioComputing, 1996), he asks the
question "Can a single molecule possess intelligence?" In discussing
this question he suggests that because of the limited capabilities of
computers, scientists are beginning to seek inspiration from biology.
Living organisms operate with functional elements that are of molecular
dimensions and that exploit quantum and thermal fluctuation phenomena.

Biomaterials had not been
seriously considered for the construction of electronic devices until
Nikolai Vsevolodov and his colleagues first produced an imaging device
and microfilm made from biological materials called Biochrom film. The
key substance was bacteriorhodopsin. Since this first study, several attempts
to produce imaging and information storage devices using biological materials
have been published. Many of these publications have come from the laboratory
of Robert Birge at Syracuse University where he has developed a three
dimensional information storage device that incorporates bacteriorhodopsin
as the storage element.

With the availability of self-assembling
membrane systems (SAMs) the stage has been set for the rapid development
of biomolecular electronic devices and their assembly using SAM type technologies.
As an example, it is obvious that a biological motor cannot be assembled
in any way that could be commercially viable other then through a self-assembling
process.

Biological molecules, particularly
proteins and lipids have all the basic properties necessary for the assembly
of nanoscale electronic devices. These biological materials conduct current,
transfer molecules from one location to another, are capable of major
color changes on application of current or light and can produce cascades
that can be used for amplification of a optical or electronic signal.
All of these properties can be applied to electronic switches, gates,
storage devices, biosensors and biological transistors to name just a
few.

The following white paper prepared
by Dr. Steven Kornguth, University of Texas is an attempt to look at biomolecular
electronics as a technology or group of technologies ready for exploitation.

After reading this document, comments
and additional papers would be most welcome. Those that add to the present
white paper will be added to the website for further reading and discussion.
It is the hope of ATP that commercial firms and their partners both in
industry, government and academia will consider the possibilities of this
technology area for further research and development. ATP looks forward
to further discussions of this topic and to proposals that suggest applications
that will lead to commercialization. Further detailed information is available
on this ATP website.

Nanotechnology and Biomolecular Electronics

The rapid advances experienced
during the past two decades in biotechnology, electronics and computer
systems provide new opportunities for convergent technology development
utilizing all three sectors. One of the major developments in biotechnology
has been the characterization of the structural/functional correlates
of biopolymers. This white paper addresses the potential uses of biopolymers
as self-assembling monolayers, as electronic and photonic conductive elements
and as molecular motors. Interest in the material properties of biopolymers
arises from 1) their self assembly properties; 2) their low cost of production
in single cell growth chambers; 3) the environmental compatibility of
aqueous systems used for biopolymer production in cell culture and 4)
the ability to use genetic engineering for transfer of genes to bacteria
or plants. The self assembling monolayers can serve as matrices for electronic
and photonic conductive elements having 1-10 nanometer thicknesses and
mm lengths. Biopolymers can function as transducers of light to electric
pulses (photon/electron transducers) with applications in information
storage and retrieval. Some biopolymers function as molecular motors having
dimensions on the 10 nanometer scale. The technological challenges that
must be overcome for cost effective production of end items will also
be considered.

Biopolymers and biomimetics have
several advantages over many other materials. Many have evolved with the
capacity to self-assemble into organized structures. The dimensions of
the biopolymers and of functional biopolymeric assemblies (e.g. molecular
motors, electron/photon conducting elements, light transducers) are on
the nanometer scale (10s of Angstroms). The instructions for synthesizing
biopolymers resides in the genetic code (DNA) of living organisms. Current
genetic engineering tools allow specific DNA sequences from one organism
to be inserted into another organism thereby resulting in large scale
production of such materials in cell growth chambers/fermenters and in
aqueous systems that are environmentally compatible.

Electron and photon conducting
biopolymers have been selected for properties of self- assembly in membrane
systems. Biological organisms convert the energy released by oxidation
of foodstuffs (protein, lipid and carbohydrate) into a common energy rich
compound, adenosine triphosphate (ATP), by a process involving electron
transfer in and proton transfer across membranes of mitochondria. The
polymers involved in the electron transfer processes are proteins containing
a functional heme (i.e. metallo porphyrin) group. These proteins are embedded
in phospholipid bilayers that provide boundaries for the transfer of electrons
and protons in a vectoral manner.

The primary challenges to be overcome
include the need to:

identify the location of each
receptor or electron/photon transfer element on a sensor so that operational
systems may be produced with high reliability;

absorb biopolymeric assemblies
onto solid matrices with retention of the functional properties of the
polymers;

retain uniform thicknesses
of biopolymers on the solid matrix of a system so that the efficiency
of the system is predictable.

Self-Assembling Monolayers

Self assembling monolayers
(SAM) can be prepared using biopolymers deposited in an ordered manner and
uniform thickness on elastomers, silicates, gold or other metallic monolayers.
One method for the preparation of SAMs involves reacting omega thiol alkane
carboxylic acids with a monolayer of gold deposited on a stable matrix
(Jordan, Frey, et al. Langmuir 10: 3642-3648 1994; Frey, Jordan et al.
1995 Analytical Chemistry 67: 4452-4457 1995; Kornguth, Corn, et al. U.S.
Patent No. 5629213. May 13, 1997. US Patent Office). A co-valent
linkage forms between the thiol functional group and gold. A uniform carpet
of carboxylated fatty acids is generated creating a polyanionic surface.
Polycationic compounds (e.g. polylysine) may then be deposited electrostatically
on the carpet, resulting in a uniformly coated surface with amine functional
groups bound to the polyanionic lipid surface and exposed to the surface.
Approximately two amine functional groups are bound to the lipid surface
for each amine facing the aqueous solution. The free amino groups (i.e.
not bound to the surface) may be coupled to oligonucleotides, antibodies,
other proteins, porphyrins/phthalocyanins. The resulting antibody and oligonucleotide
platforms may serve as sensors for biomedical, biodefense or environmental
monitoring. The proteins/porphyrins/phthalocyanins that are bound to the
surface may function as electron/photon conductive elements with nanoscale
dimensions (Ostuni and Whitesides. Colloids and Surfaces B-Biointerfaces
15: 3-30 1999). One advantage of the thiol alkane-gold complex is the lability
of this bond to uv light. With appropriate masks, it is possible to prepare
patterned arrays where each single element contains a specific binding entity.
One product of this strategy is a multi-array sensor or optical read/write
disc. A second product may be a nanoscale electronic/photonic circuit with
attendant benefits of miniaturization, low power requirements, high efficiency,
low heat generation. Self assembling biopolymers, that form right or left
handed helical structures, can be produced. In some cases, the addition
of ions changes the helicity of these nanostructures with resulting applications
for optoelectronic devices including sensors (Engelkamp, Middelbeek and
Nolte Science 284 785-788 1999)

Optical detection systems have
been developed using SAMs. One optical detection system relies on surface
plasmon resonance (Kornguth, Corn, et al. U.S. Patent No. 5629213. May
13, 1997) to detect changes in the thickness of biopolymers covering a
set of thin films comprised of glass, thiol alkane, polylysine and specific
high affinity molecular binders. A second utilizes fiber optics coated
with a film containing antibodies, oligonucleotides, or receptors. The
binding of target to a precoated fiber results in changes in the intensity
of light emerging from the evanescent wave. The KD of the binding
agents for selected targets approximates 10-8 for antibodies
and 10-14 for certain receptors. Amperometric sensor systems
have been constructed utilizing SAMs coated with high affinity binders
and an electron conducting biomimetic such as polypyrrole (Cosnier, Stoytcheva
et al. Anal. Chem 71:3692-3697 1999). The advantages of the SAMs is the
low cost of production, and the capability of recycling gold coated glass
wafers following irradiation with uv light (uv light dissociates thiol
alkane from the gold). The disadvantage is the necessity to maintain alignment
of a light source with the surface containing the specific binder (i.e.
antibody, antigen, oligonucleotide, toxicant). While not a difficulty
in clinical laboratories, this may present a challenge to utilization
of the technology in field situations. The development of monolithic sensors,
having a light emitter and detector on a common plane, in a microenvironment
separated by binder and target, would facilitate construction of a field
hardened end item.

Electron/Photon Conductive Biopolymers
and Nanotubes

Several biopolymers have well
documented properties as organic electron conductors. These materials,
exemplified by the cytochrome systems, have tetrapyrrole components (porphyrins)
that are usually metal centered. The tetrapyrrole is a highly conjugated
system that can interact with other tetrapyrroles in a face to face orientation
with P bonding. Using model systems Collman and colleagues (J Amer
Chem Soc 102:6027-6036 1980) has demonstrated that electron transfer is
maximized when the face-to-face distances are maintained at 5-8 Angstroms.
Electron transfer may be mediated both through P stacking and redox
of the metal center. Phthlocyanins are biomimetics of porphyrins and these
have been shown to exhibit modest electron conductivity when doped (Marks,
Science 227:881-889 1985). Amperometric sensors have been constructed
utilizing biotinylated polypyrroles (Cosnier et al. Analytical Chemistry
71: 3692-3697 1999) and proteins containing porphyrins (Mizutani, Sato
et al. Electrochimica Acta 44: 3833-3838 1999). A challenge presented
by this technology is the production of filaments of the heme or phthalocyanine
entities in most efficient alignment for electron transfer. The development
of biopolymer based molecular switches enable more rapid development of
molecular transistors and integrated circuits.

Nucleic acids can function both
as organic electron transfer materials and as templates for the deposition
of electron conducting metals. The rate of electron transfer through organic
conductors is approximately four orders of magnitude slower than through
good metallic conductors. Double stranded deoxyribonucleic acid (dDNA)
has now been demonstrated to function as an organic electron transfer
material ("wire"). The electron transfer is effected through
stacking and orientation of the bases (SO Kelley, JK Barton. Science 283:
375-381 1999; Wan, Fiebig et al. Proc Natl. Acad. Sci US, 96: 6-14-6019,
1999; Henderson, Jones et. al Proc. Natl. Acad Sci US, 96: 8353-8358 1999).
DNA has also been shown to serve as a matrix for adsorption to gold or
silver in the construction of nanowires and sensors (Elghanian, Storhoff
et al. Science 277: 1078 - 1081 1997; Braun, Eichen et al. Nature 391:
775 1998). The nanowires are capable of electron conduction as metallic
materials. The problems associated with this technology include the formation
of uniform diameter and oriented polynucleotide fibers. Methods have yet
to be developed for production of an ordered deposition of the "nanowires"
(DNA or DNA gold complex with deposited metal)" on a support surface.
An end product would be a nanoscale integrated circuit.

Nanotubes have been formed using
organic polymers as templates (Rudolph AS, Ratna BR and Kahn B. Nature
352: 52-55 1991). The nanotubes have diameters of 1 nanometer or larger
and have utility as molecular tweezers or surface probes (Gimzewski and
Joachim Science 283 1683 1999). The molecular tweezers enable one to move
single molecules on a solid surface for the construction of sensors and
integrated molecular motor systems (Kim and Lieber Science 286: 2148-2150
1999; Baughman, Cui et al. Science 284: 1340-1344 1999). These structures
may also be used to map the surface properties (i.e. uniform thickness,
electrical conductivity, force required to separate two biomolecular complexes)
of thin films (Gimzewski and Joachim Science 283 1683 1999). The nanoscale
dimensions of the tubes, their physical strength and electronic conducting
properties have utilities in a variety of industries including electronics,
biomedicine, communications and QC in the manufacture of thin films. The
technical issues to be addressed include mass production of uniformly
thick tubules, the deposition of the tubules in an ordered manner and
attachment of the tubes to larger electron conducting surfaces.

Molecular Motors

Living systems require motor devices
and energy sources for normal function. Examples of such motors are the
protein molecules kinesin and dynein (Howard, Hudspeth and Vale Nature
342 154 1989; Block, Goldstein, Schnapp et al. Nature 348: 348 1990) that
serve to propel particles from one portion of a nerve cell to another
(the distance to be traversed may be longer than 1 meter), the protein
F1-ATPase (Stock, Leslie and Walker Science 286: 1700-1705 1999; Noji
Science 282 1844-1845 1998), myosin (Mermall, Post and Mooseker Science
279 527-533 1998) and RNA polymerase (Yin et al. Science 270: 1653 1995;
Mehta, Rief et al. ibid). The energy used to drive the motors is present
in adenosine triphosphate (ATP), a molecule whose synthesis in mitochondria
is coupled to the oxidation of amino acids, carbohydrates and fats. The
molecular motors can provide forward propulsion or rotational movement
in a preferred direction; these are vectorial in nature (Mehta, Rief,
Spudich et al. Science 283: 1689-1695 1999). The direction of the movement
by kinesin and dynein are determined by the orientation of tubulin, the
molecular matrix that the motors use as tracks

For in vitro applications
of molecular motors, some estimate of the force generated is useful. Kinesin
moves in steps of 8-16 nm along a tubulin track and movement stalls at
loads between 5-7 pN (Mehta, Rief et al. Science 283: 1689-1695 1999).
The movement of the kinesin is ATP dependent. Whereas movement of the
load slows as a function of increasing load, the rate of ATP hydrolysis
does not; this suggests that the increase in load reduces the probability
of a mechanical step. The rotational torque required for movement of the
F1 ATPase is about 80 pN nm and the energy available from the hydrolysis
of a single ATP approximates 100 pN. Therefore the efficiency of this
system is high (Noji Science 282: 1844-1845 1998). The actin-myosin system
step size is about 10 nm and similar force fields have been identified.
The stall force required for stopping the RNA polymerase movement is on
the order of 25 pN (Wang et al. Science 282: 902 1998). The RNAP step
size is estimated at about 1 base pair separation. All these measurements
suggest that the forces involved in the ATP dependent movement of molecular
motors are within 1 order of magnitude and are highly efficient. These
motors have applications in the design of switching devices and nanometer
scale rotary devices.

The scale of the nanomotors (about
2-20 nm) is three orders of magnitude smaller than the advanced micromachined
motors (75 microns) generated by X-ray lithography and surface micromachining
(H Guckel at the University of Wisconsin-Madison). The biopolymer and
biomimetic motors have certain advantages and disadvantages as compared
with the LIGA machined devices; novel grafting and SAM technology may
however now result in hybrid biomolecular-machined motors. In the hybrid
motor, the biopolymer could serve as a light tube switch. The biopolymer
would not be expected to control movement of the machined motor to the
nanoscale level.

Conclusion

Biopolymers and biomimetic polymers
have several materials properties of interest to the industrial sectors
of electronics, optics, pharmaceutics. The materials have applications
in the design and construction of nanoscale integrated circuits, of laminated
structural elements (smart sensor interiors in automobiles, planes and
trains), of microsensors that can be embedded in persons or animals or
placed on protective clothing. The realization of these opportunities
requires solutions to some of the challenges identified in this paper.

Opportunities

The industries affected by the
technologies identified above include pharmaceuticals, medicine, food
processing, cosmetics, electronics, communications, transportation. The
end devices that are envisaged at this time include multi-array sensors,
electronic/photonic integrated circuits, read/write discs. The components
of devices include molecular light/electron switches, light valves, transistors,
drug delivery vehicles. The time is right for the rapid development and
exploitation of these new and exciting technologies.